An image formation apparatus such as the laser printer generally forms images on paper sheets, which are paper-like media of various types being different in character from each other, such as heavy paper, copy paper, OHP films. In such an image formation apparatus, the various conditions of the printing/fixing process may be optimized in accordance with the type of each paper sheet to be used, in order to form images of high quality. To optimize the various conditions of the printing/fixing process, the apparatus needs parameter data on the type of a paper sheet, such as the thickness, density and grammage. Hitherto known is an image formation apparatus including a console panel, which the user may operate to designate the type of a paper sheet. In recent years, a sensor called “media sensor” has come into use. The media sensor automatically determines the type of a paper sheet. In any image formation apparatus that includes such a sensor, the type of a paper sheet is determined without the user's manual operation, whereby the conditions of forming images are optimized.
Various methods of determining the type of a paper sheet have been proposed for use in image formation apparatuses. JP-A 7-196207 (KOKAI) discloses a method in which a sensor unit provided on a conveyance path applies light to every paper sheet being conveyed and measures the thickness and density of the paper sheet based on the light transmittance of the paper sheet, whereby to determine the type of the paper sheet. In this method, the type of any paper sheet is determined after the conveyance of the paper sheet has been started. However, if the type of any paper sheet is determined after the start of paper sheet conveyance, the conditions of the printing/fixing process, such as the temperature of the fixing drum, cannot be set in time because the speed of forming images has increased in recent years.
JP-A 2003-226447 (KOKAI) and JP-A 2005-104723 (KOKAI) disclose methods, in which the data, such as the thickness of each of paper sheets, is acquired before the paper sheets are conveyed, or while the paper sheets remain in the sheet feed tray of the image formation apparatus. In the method disclosed in JP-A 2003-226447 (KOKAI), one side surface of a pile of paper sheets which are stacked is imaged, an inter-peak distance in the waveform with the unevenness defined by the paper sheets is then calculated, and the thickness of each paper sheet is calculated. In this case, a light source that operates in unison with an image sensor applies illumination light to the side surface slantwise from above or below in order to accentuate the subtle irregularities on the side surface of the pile of the paper sheets. In the method disclosed in JP-A 2005-104723 (KOKAI), a waveform with the unevenness in one side surface of a pile of paper sheets is acquired in the same way, and a frequency analysis such as fast Fourier transform is performed to calculate the thickness of each paper sheet.
These methods, in which a side surface of a pile of paper sheets is merely imaged, can provide only data, e.g., the thickness of each paper sheet and the number of paper sheets. In order to find the grammage of each paper sheet, it is required to detect the density of the paper sheet in addition to the thickness of the paper sheet.
As described above, in the method of JP-A 7-196207 (KOKAI), the conditions important in printing, such as the temperature of the fixing drum, cannot be set in time because the type of any paper sheet is determined after the start of paper sheet conveyance. In the methods of JP-A 2003-226447 (KOKAI) and JP-A 2005-104723 (KOKAI), the type of paper sheets can be determined while the paper sheets remain in the sheet feed tray, but the data acquired is only about the thickness of each paper sheet and the number of paper sheets.
In the image formation apparatus, it is required to acquire parameter data, such as not only the thickness of each paper sheet but also the grammage thereof and determine the type of the paper sheet for forming an image of high quality on the paper sheet.
Therefore, in a method of determining the type of a paper sheet, it is required to reliably determine the type of the paper sheet at high precision.